RFR: 8283737: riscv: MacroAssembler::stop() is used in AD file and it should generate a fixed-length size

[Xiaolin Zheng]

Hi team,

Could I have a review of this simple patch? -

`PhaseOutput::fill_buffer` detects if the real size of a node matches (<=) the size of it in scratch_emit(). The call chain for MacroAssembler::stop() is:


MachEpilogNode::emit
    -> reserved_stack_check()
         -> should_not_reach_here()
              -> stop(const char *msg)


`li()` on RISCV could generate 1~6 instructions, and the msg argument could be an on-stack buffer; `stop()` also uses `__ pc()` that could also be different in `scratch_emit()` and `emit()`. They both have the potential issue here so the size generated in `MacroAssembler::stop()` needs to be a fixed value.

Could be reproduced in the fastdebug build by adding one line:


// Die now.
instruct ShouldNotReachHere() %{
  match(Halt);
  ins_cost(BRANCH_COST);
  format %{ "#@ShouldNotReachHere" %}
  ins_encode %{
    Assembler::CompressibleRegion cr(&_masm);
    if (is_reachable()) {
      __ halt();
+     __ unimplemented("this is an on-stack char literal");  // assertion fail at 'assert(false, "wrong size of mach node");'
    }
  %}
  ins_pipe(pipe_class_default);
%}


This patch also fixes a typo introduced in JDK-8278994: `c_bnez` is mistakenly written to `c_beqz`, though not used until now, needing a fix for future usage.


Tests passed in hotspot tier1 & jdk tier1 without new errors found.

Thanks,
Xiaolin

-------------

Commit messages:
 - Fix a typo instroduced when refactoring: c.beqz
 - MacroAssembler::stop() is used in adfile and it should generate a fixed-length size

Changes: https://git.openjdk.java.net/jdk/pull/7982/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7982&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283737
  Stats: 5 lines in 2 files changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7982.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7982/head:pull/7982

PR: https://git.openjdk.java.net/jdk/pull/7982